Process of etching a shadow mask

ABSTRACT

A SHADOW MASK FOR A COLOR TELEVISION TUBE, HAVING A PATTERN OF APERTURES DIMENSIONED AS REQUIRED FOR SCREENING, IS FURTHER ETCHED AFTER THE SCREENING PROCESS TO ATTAIN A HOLE SIZE LARGER THAN THE PHOSPHOR DEPOSITS OF THE SCREEN. THE RE-ETCHING OCCURS IN SEVERAL STAGES AND A DENSITOMER ASSOCIATED WITH ONE SUCH STAGE MEASURES THE HOLE SIZE AND DERIVES A VOLTAGE USED TO CONTROL THE ETCHING TIME SO THAT THE APERTURES OF THE RE-ETCHED MASK ARE PRECISELY CONTROLLED TO A DESIRED SIZE.

June 13, 1972 L. LERNER $669,771

PROCESS OF ETCHING A SHADOW MASK Filed Jan. 28, 1970 (l4 68BX82ProporTi0n0I r I2 lomo F l Oxide o o o D e 151. c 0 2nd. c 0 5rd. c D4th 0 o ecor omze FEZZ Swge stage W T Bloc ken.

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(Time) Size (Fe Hole Size (Dry) lnvenror Morhn L. Lerner Attorney AirSource UnitedStates Patent Office Patented June 13, 1972 3,669,771PROCESS OF ETCHING A SHADOW MASK Martin L. Lerner, River Forest, Ill.,assignor to Zenith Radio Corporation, Chicago, Ill. Filed Jan. 28, 1970,Ser. No. 6,619 Int. Cl. C23f 1/02 US. Cl. 156-8 9 Claims ABSTRACT OF THEDISCLOSURE A shadow mask for a color television tube, having a patternof apertures dimensioned as required for screening, is further etchedafter the screening process to attain a hole size larger than thephosphor deposits of the screen. The re-etching occurs in several stagesand a densitometer associated with one such stage measures the hole sizeand derives a voltage used to control the etching time so that theapertures of the re-etched mask are precisely controlled to a desiredsize.

BACKGROUND OF THE INVENTION The subject invention is directed to aprocess of etching a pattern of apertures of a desired size in anelectrode, such as the color-selection electrode or shadow mask, of thecurrent form of shadow-mask color picture tube. The process may beemployed to develop an aperture pattern in a mask blank in the initialsteps of fabricating the mask and it has equal application to what hasbecome known as etch back. Etch back is that step in the fabrication ofa color television picture tube in which an aperture mask that has beenemployed in photographically printing a phosphor mosaic on the screen ofa cathode-ray tube has its holes enlarged to the end that the crosssection of the electron beams of such a tube, as determined by theapertures of the mask, exceeds the size of the phosphor deposits on thescreen. This relation of beam to dot size is characteristic of both ablack-surround and a postdeflection-focus color picture tube.

A black surround color tube is the subject of Pat. 3,146,368, issuedAug. 24, 1964, in the name of Joseph P. Fiore et a1. and assigned to theassignee of the present invention. It differs from conventional shadowmask types of cathode-ray tubes in two material respects: (1) eachphosphor dot of the screen is surrounded by a material that is absortiveof light, and (2) its electron beam diameter is larger than the diameterof the phosphor dots. A post-deflection-focus or acceleration color tubediffers from the conventional shadow mask device in that additional beamfocusing is introduced after the center of deflection. Because of theadded focusing, more of the beam electrons are able to impinge upon thescreen than otherwise and it is necessary that the phosphor dots besmaller in size than the apertures of the mask. In the latter respect,it is much like the black-surround tube.

Difficulties have been encountered in the screening of these types oftubes and best results for production on a commercial scale have beenachieved with an etch back process. In that process, the shadow mask isformed and contoured in the conventional way, differing only as to holesize. The apertures of the mask initially have the dimensions requiredfor screening so that the mask may be utilized in conventional mannerfor photographically printing the green, blue and red phosphors on thescreen and, in the case of black surround tubes, for developing holes ina light-absorbing layer covering the screen area and into which phosphoris to be deposited. When this has been accomplished, the mask issubjected to another etching process, namely etch back, in order toenlarge or open up its apertures to the size required for the mask as itis to be finally installed in the tube envelope.

The general concept of this etch back process is, of course, wellunderstood; it simply entails re-introducing the mask into a workstationwhere it is immersed, sprayed or otherwise treated with an etchantsolution of sufficient concentration and for a suitable period of timeto etch away the walls of the apertures and enlarge them to the sizerequired. It will be appreciated that the final size of the apertures ofthe mask is critical. If they are too large, problems of color fieldpurity will be experienced with the tube in which the mask is installed.On the other hand, if the apertures are too small in conjunction withmisregistration attendant problems of white field purity will occur.

Previous efforts have been made to facilitate precision of the etch backprocess. For example, copending application Ser. No. 811,318, filed Mar.28, 1969, in the name of Sam H. Kaplan discloses improvements in theetch back process attained by contouring or shaping the holes asoriginally developed in the shadow mask. In another copendingapplication, Ser. No. 850,408, filed Aug. 15, 1969, in the name ofJoseph M. Black, still further improvements are attained by arrangingthe apertures of the mask uniquely to reduce the processing time of theetch back step. Both of these applications are assigned to the assigneeof the present invention.

While these approaches are attractive and useful, the present inventionis a still further development which makes possible precision of theetch back process whatever may be the specific shape of the aperturesinitially developed in the mask. Accordingly, it is an object of thepresent invention to provide an improved process for etching a patternof apertures in an electrode or shadow mask.

SUMMARY OF THE INVENTION The process of the invention is for etching apattern of apertures of predetermined size in an electrode formed of amaterial that is subject to attack by a particular etchant. The processcomprises the. following steps. Initially, the electrode is subjected,while in a reference condition, to a solution of that etchant for afirst etching interval to develop the desired pattern of apertures, ifthe electrode does not have such an aperture pattern in its referencecondition, or alternatively to enlarge the apertures if the electrodedoes have such a pattern in its reference condition but in either casethis etching step limits the aperture size to a value which is less thanthe predetermined size ultimately desired. A stream of particles isdirected upon the surface of the electrode after the first etching stepand a control effect is derived therefrom representing the instantaneoussize of the apertures as reflected by their transmissivity to suchparticles. This control effect is utilized to control a second andsubsequent etching of the electrode with the etchant to develop theapertures of the pattern substantially to the desired predeterminedsize.

In one aspect of the invention, the electrode is subjected to an etchingsolution in one stage for a particular time and at the end of that timea control potential is developed which is a measure of the response ofthe material of the electrode to the etching process. This controlpotential is used to adjust the processing time of one or moresucceeding stages in which the electrode is treated with an etchingsolution, preferably the same as that employed in the first stage. Thecontrol permits precision in the final size of the apertures of theelectrode.

As indicated above, the structure to be etched is variously referred toas a color-selection electrode, shadow mask or aperture mask employed ina color cathode-ray tube so that, in the three-gun variety, the electronbeam issued from each of the three guns is permitted to excite phosphorsof only an assigned one of the three colors constituting the mosaic ordot triad type of screen. For convenience hereinafter, the structurewill be referred to as an aperture mask and also for convenience it willbe assumed that the screen is of the type which has circular or dotshaped deposits of phosphor obtained by photographic printing with anaperture mask having a pattern of circular holes arranged in a fieldcorresponding to the shape of the image screen of the tube. The aperturefield may be circular or rectangular but usually a rectangular patternis employed.

It will also be appreciated, especially as the details of the inventiveprocess'are described, that the specific configuration of the aperturesof the mask is of no particular consequence. Usually, they are roundalthough they may be hexagonal or rectangular. Any such aperture maskmay have its aperture pattern developed and/or enlarged to preciselycontrolled dimensions through the practice of the subject invention.

BRIEF DESCRIPTION OF THE DRAWING The features of the present inventionwhich are believed to be novel are set forth with particularity in theappended claims. The invention, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawing, in theseveral figures of which like reference numerals identity like elements,and in which:

FIG. 1 is a block diagram of apparatus for performing the inventiveprocess;

FIG. 2 is a schematic representation of a densitometer arrangement thatmay be employed in one or more of the etching stages indicated in thearrangement of FIG. 1; while FIGS. 3 and 4 are curves used in explainingthe process conducted with the arrangement of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Further to simply thedisclosure, the invention will be described initially in the environmentof re-etching or etch back and it will be assumed that an aperture maskhaving a rectangular pattern of circular apertures has been employed forphotographically screening the cathode-ray tube in which the mask isultimately to be installed. The mask will, therefore, be conventionalexcept that its apertures will have the dimensions required forscreening. They may have uniform size or may be graded, being a littlelarger in the center than at the edges. In other words, the holediameter decreases gradually with radial distance from the center of themask. The mask stock is generally cold rolled steel of approximately 7mil thickness with. apertures in the center having a diameter of about 9mils. The mask will also have been oxidized or blackened on bothsurfaces to exhibit the heat-conducting and the lightreflectingproperties of a black body. This surface treatment is distinctlypreferred where the aperture mask is employed in photographic screeningbut since the details of that screening are of no moment to the presentinvention, they need not be considered further herein.

The oxide coating tends to resist the etchant otherwise employed informing the aperture pattern in the mask and, therefore, the firstworkstation 10 of the arrangement of FIG. 1 is an oxide stripper andrinse. In this station, the oxide coating on the flat surfaces of themask and on the walls of its apertures is removed by a treatment withhydrochloric acid and a detergent and after the mask has been strippedof its oxide coatings, it is rinsed with water.

After leaving workstation 10, the mask is in its reference or startingcondition by which is meant for the assumed embodiment of the inventionthe condition of the mask just prior to etch back. It, therefore, doeshave a desired aperture pattern that was previously developed in theinitial fabrication of the mask. The next step in the process isconducted in a multistage etching system 11,

shown as comprising four stages lla-lld although the number of stages isof no particular importance. Preferably, the mask is subjected to thesame etching solution and process in each of the four stages which mayindividually be of any well known structural arrangement. By way ofillustration, etching apparatus of the general type that may be employedis disclosed in Pat. 2,762,149, issued Sept. 11, 1956 and Pat.2,822,635, issued Feb. 11, 1958, both in the name of N. B. Mears. Sincea multiplicity or succession of etching stages are contemplated, it isconvenient to have the mask carried by a step-by-step conveyor througheach of the several workstations.

Each etching stage is a chamber which may have entrance and exit doorsautomatically operated by a suitable programmer to permit an aperturemask carried by the conveyor to be introduced into the chamber and to beremoved therefrom after a chosen time interval. In each chamber there isa cluster or field of spray heads positioned with respect to the restposition of the mask in that stage to direct a uniform flow of anetching solution of suitable concentration over one or both surfaces ofthe mask. Of course, the solution includes an etchant which attacks themask material and may, for example, be ferric chloride. Best results canbe expected if all four etching stages receive etching solution from acommon source so that the parameters of the process, as imposed by theetching solution itself, are common to all stages. The final stage 11dleads to an arrangement 12 where the re-etched mask is rinsed,de-carbonized, blackened and finally rinsed with de-ionized water. It isfound that a mask formed of cold rolled steel may have a carbon film asit emerges from the re-etch stations; this film is removed by atreatment of phosphoric acid. Blackening is undertaken to restore anoxide coating to the surfaces of the mask because it is desirable thatthe mask have the heat-conducting and light-reflecting properties of ablack body as finally installed in the tube. The blackening may beaccomplished by heat treatment in an oxidizing atmosphere or this may beaccomplished chemically in a salt bath of iron or zinc phosphate. Thesesteps are well known and of themselves constitute no part of the present invention. As thus far described, the re-etching arrangement of FIG.1 is conventional; consideration will now be given to the improvementintroduced by the present invention.

More particularly, the process of the invention involves directing astream of particles upon the surface of the mask after a predeterminedetching time and deriving therefrom a control effect representing thesize of the apertures in the mask as reflected by its transmissivity tosuch particles. The nature of the particles employed in deriving thedesired control effect is subject to considerable variation and may besolid particles or energy particles, that is to say, utilizing theparticle analogue of energy as is frequently done especially in dealingwith photons and phonons. It is convenient and practical to use lightenergy, visible or invisible but, of course, sources of visible lightquickly suggest themselves. An appropriate arrangement for responding tolight to develop a control voltage is referred to in the art as adensitometer and is indicated schematically in FIG. 2 which representsetching stage wherein a mask 20 in process is shown supported on acarriage 21 in the form of an open frame secured to or constituting partof the work carrying conveyor and through which light from a source 22collimated by a colliamtor 23 may be directed toward the central area ofthe mask. A photocell 25 is positioned directly across from light source22 at workstation 110 to intercept and measure the amount of lightenergy transmitted through the mask apertures that are interposed in thelight path. It is preferred that the light beam be large in crosssection compared to the area of the individual apertures of the mask, across section of one square inch being representative of a useful beamsize. Air is blown through nozzle 24 to clear away adhered acid.

It is convenient to mount photocell 25 within and above the cluster ofspray heads 30 through which etching solution delivered from a source(not shown) is admitted through a valve 31 and header 32 to the variousspray heads.

The photocell develops a control effect, specifically a control voltage,dependent upon the quantity of light incident thereon and this controleffect is utilized to control the etching of the mask in process inorder to develop the apertures thereof to the desired size. Calibrationof the densitometer to effect control of the etching process so that there-etched mask has precisely dimensioned apertures is readily obtainedby using, as a reference, the value of voltage developed either when thelight path between source 22 and photocell 25 is unencumbered by a maskor in the presence of a standard mask known to have apertures of theproper dimensions. In the pres ence of a mask having smaller dimensions,less light impinges upon cell 25 and the voltage developed isessentially a linear function of incident light. Accordingly, thevoltage output of cell 25 represents the measured aperture size of themask in process and may be utilized to control any subsequent etchingthat may be necessary to achieve the desired hole size in the mask. Thisdensitometer arrangement for developing a voltage to control the re-etchprocess is represented in FIG. 1 by the box 13.

The control voltage developed in unit 13 is applied to a proportionalcontroller 14 associated with etching stage 110 to adjust at least oneparameter thereof for the purpose of controlling the re-etch process.For the simplest and preferred case, the processing time is controlledsince it is desired that the etching solution of all four stages be thesame. Unit 14 in response to the control voltage from unit 13 developsan output signal having a duration which varies with the magnitude ofthe applied control voltage. The signal output of unit 14, bycontrolling an electrically operable valve such as valve 31 of stage110, adjusts the etching time of that stage. Proportional controllers ofthis type are known in the art and a solid state form is available fromthe Potter Brumfield Division of American Machine & Foundry Companyunder the designation of a time delay relay.

The process carried out in the arrangement of FIG. 1 is one in which theaperture mask, having been stripped of its surface oxide layers atworkstation 10, is etched successively with a common etching solutionand for equal and known time intervals in stages Ila-11b. The mask isthen transferred to stage 110 and, as is common practice with start-stopconveyor systems, a rest time is available in which the doors preferablyclose over the entrance and exit ports of the stage. Also during thistime lamp 22 is energized and directs a beam of light through aperturemask 20 to photocell 25. Since the starting conditions of the mask areknown, because the mask has apertures of preselected size for use inscreening, and since the parameters and process times of stages 11a and1117 are likewise known, the reading of the densitometer or themagnitude of control voltage developed in source 13, reflects theresponse of the mask blank to the etchant and it further represents thesize of the apertures in the mask as reflected by the transmissivity ofthe mask to light. It is preferred that the processing times of stages11a and 11b be insufiicient to achieve an aperture size that is requiredof the mask in its final form; certainly, the final aperature size mustnot be exceeded in these stages. If this condition is satisfied, thedensitometer measurement in determining the speed of the etching processin respect of the mask instantaneously in process further determines theamount of continued etching, if any, required to attain the desired sizeapertures in the mask. This determination is manifest in the voltageapplied from source 13 to proportional controller 14 which accordinglyadjusts the etching time of stage 110. If desired, the control of unit14 may also extend to the fourth stage 11d of the etching arrangement asindicated. Since the control voltage is developed in unit 13 beforeetching is initiated in stage 110, control unit 14 may adjust the timeof the third stage. If inadequate processing time is available in stage11C, further control of the etching may take place in stage 11d.

The curves of FIG. 3 reflect the variable response of mask blanks to theetching process and clearly demon- Strate the need served by the presentinvention. Curve A indicates a mask response that attains an aperturesize within an acceptable tolerance range, represented by ordinatelimits Min and Max in the process interval of the third stage 110. Inthis instance proportional controller 14 interrupts the etching of thatstage within these limits. For the mask represented by curve B anacceptable aperture size is not obtained until the fourth stage 11d isreached and in this case controller 14 permits stage to operate over itstotal available etching time and adjusts stage 11d to terminate etchingtherein at the appropriate time. The response of the mask material tothe etching process may vary over such wide limits that the condition ofcurve C can be expected to be experienced. This represents the casewhere an aperture size Within acceptable limits is attained at theconclusion of the etching process of stage 11b. In such a case thecontrol voltage from source 13, in operating upon controller 14,prevents additional etching of this particular mask in stages 11c and11d.

Of course, the densitometer measurement may be made after the mask inprocess has been wiped dry, removing all solution and liquid from themask. It is distinctly preferable, and has been found in fact verypracticable, to conduct the densitometer measurement while the mask isstill wet. Experiments have shown that consistent densitometer readingsmay be made after the mask has been wetted by a liquid of such surfacetension as to form a film or lens over the mask apertures so long as thesolution is at least partially transparent to the light beam employed inthe densitometer. For example, acceptable and consistent readings may bemade after the mask has been etched and rinsed with water. It has alsobeen found, however, that equivalent results may be obtained if themeasurement is made directly after etching and while the mask is stillwet with etching solution. The curve of FIG. 4 indicates that a directcorrelation may be made between the hole size of the mask wet withetching solution and the hole size of the mask after it has been dried.Accordingly, a distinct advantage of the described process is that themeasurement may be made and the process controls adjusted withoutremoving the mask from the etch workstations.

A multistation etching system of the type shown is attractive for massproduction but the invention is not limited in this respect. It may beutilized advantageously where a single workstation is used. In thatcase, the mask is etched for a partciular process time, measured andthen subjected to additional etching at the same workstation ifnecessary to achieve a precisely controlled aperature size.

It has been convenient to describe the invention in the environment ofre-etch or etch back simply because the circumstances of that processgave rise to the described solution of precisely controlling the etchingprocess in order to size the apertures of the mask for a black surroundtube. The process is, of course, suitable for forming a pattern ofapertures in a mask blank while it is in its flattened, imperforatesheet form. This is usually accomplished by arranging resist coatings onboth surfaces of the mask with interruptions or openings that representthe apertures to be formed. In such case, the etching process isconducted for a sufiicient time interval to at least develop a desiredpattern of holes in the mask after which the densitometer measurement istaken to control the further etching in order that the apertures mayhave a precisely controlled final size.

Prior to the development of the process control described herein, theexperience of etch back in particular was a variation in hole size ofthe masks as much as 5 mils.

With the process control of this invention, the tolerance range has beendrastically reduced and may conveniently be kept within a range of .2 to.5 mil. Moreover, a most attractive saving in cost is made possible inthe preferred process of the invention because the densitometer readingis made at a point in the process before the apertures of the mask willhave attained the desired final size. This avoids the objection of pastpractices wherein the measurement of hole size could frequently occurafter the maximum allowable size had been exceeded with consequentdestruction of the mask. Of course, the degree of control may be furtherextended by providing each of stages 11b, 11c and 11d with its owncontrol arrangement comprising counterparts of units 13 and 14individual to each such stage.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the apended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

I claim:

1. The process of etching a pattern of apertures of a predetermined sizein a shadow mask formed of a material that is subject to attack by aparticular etchant which process comprises the following steps:

subjecting said shadow mask, while in a reference condition, to asolution including said etchant for a first etching interval to developsaid patern of apertures, if said shadow mask does not have such anaperture pattern in said reference condition, or to enlarge theapertures if said shadow mask has such an aperture pattern in saidreference condition but in either case to attain an aperture size whichis less than said predetermined size;

directing a stream of energy particles upon one surface of said shadowmask after said etching interval and deriving from energy particlestransmitted through the apertures of said shadow mask a control efiectrepresenting the instantaneous size of the apertures in said shadow maskas reflected by the transmissivity of said shadow mask to saidparticles;

and utilizing said control effect to control a second and subsequentetching of said shadow mask with said etchant to develop said aperturesof said pattern substantially to said predetermined size.

2. The etching process in accordance with claim 1 in which said controleffect is derived at the completion of said first etching interval,while said shadow mask remains in the etching apparatus, and is utilizedto control a subsequent etching step to develop said apertures of saidpredetermined size.

3. The etching process in accordance with claim 2 in which said firstand subsequent etching steps are conducted with the same etchingsolution;

and in which said control effect is utilized to adjust the etching timeof said subsequent etching step.

4. The etching process in accordance with claim 1 in which a solutionemployed in the processing of said shadow mask has sufficient surfacetension to form a film over apertures of said pattern which is at leastpartially transparent to said energy particles;

and in which a stream of said particular energy is directed to saidshadow mask to derive said control effect.

5. The etching process in accordance with claim 4 in which said filmserves as a lens that is at least partially transparent to light;

and in which a light beam, having a cross section large with respect tothe area of the individual apertures of said shadow mask, is directedupon said shadow mask to derive said control effect.

6. The etching process in accordance with claim 2 in which said etchingsteps are conducted in at least two enclosed workstations between whichsaid shadow mask is transported;

and in which said control effect is derived after said shadow mask hasbeen transported from the first to the second of said workstations butbefore etching takes place in said second station.

7. Theetching process in accordance with claim 6 in which saidcontroleffect is derived by developing a light beam in said workstationand by directing said light beam upon the apertured surface of saidshadow mask and measuring within said enclosed workstation the amount ofsaid beam emerging through the apertures of said pattern.

8. The etching process is accordance with claim 6 in which said controleffect is utilized to adjust the etching time in the second of saidworkstations.

9. The process of enlarging to a predetermined size the apertures of anaperture pattern in a shadow mask formed of a material that is subjectto attack by a particular etchant, which process comprises the followingsteps:

subjecting said shadow mask to an etching solution including saidetchant for a first period of time to enlarge the apertures of saidshadow mask to a size less than said predetermined size;

at the conclusion of said first etching step measuring the instantaneoussize of said enlarged apertures in said shadow mask and derivingtherefrom a control effect representing the response of said shadow maskto said etching solution;

and utilizing said control eficect to determine the duration of a secondand subsequent etching of said shadow mask with said solution to furtherenlarge said apertures substantially to said predetermined size.

References Cited UNITED STATES PATENTS 2,762,149 9/1956 Mears 156-8OTHER REFERENCES Photocell Controlled Etcher, by Greene et al., IBMTech. Disclosure, pp. 582-4, vol. 10, No. 5, October 1967.

JACOB H. STEINBERG, Primary Examiner U.S. Cl. X.R.

